The present invention relates to an objective lens made of a plastic material that is used in a optical pickup apparatus, wherein, a light flux radiated from a laser light source is formed into a light spot by an objective lens on an information recording surface of an optical information recording medium through its transparent board, to record and/or reproduce information optically, and more particularly, to an objective lens wherein a first light source with a wavelength of xcex1 is used for recording and/or reproducing a first optical information recording medium, and a second light source with a wavelength of xcex2 is used for recording and/or reproducing a second optical information recording media, and corrects by a diffraction a spherical aberration that is produced by the difference between a thickness t1 of the transparent base board of the first optical information recording medium and a thickness t2 of the transparent base board of the second optical information recording medium.
With a short wavelength red semiconductor laser which has recently been put to practical use, there has been commercialized a high density DVD (digital versatile disc) that is the same in size as a conventional CD (compact disc) and has a larger capacity as an optical information recording medium. When information is recorded on this DVD or reproduced from this DVD, a semiconductor laser with a short wavelength of 635 nm or 650 nm and an objective lens with a numerical aperture (NA) of an approximate value of 0.6 are necessary.
Further, with a spread of a readable optical information recording medium CD-R (write-once read-many compact disc), an optical pickup apparatus is required to have the so called interchangeability of information recording medium that makes it possible to record and/or reproduce information for both CD-R and DVD.
However, since the necessary signals (reproducing signal, focus error signal and tracking error signal) cannot be obtained by a drop in the reflectance of CD-R in the short wavelength band, the semiconductor laser of wavelength 780 nm is prepared for CD-R, separately from the short wavelength semiconductor laser for DVD,.
Incidentally, for recording and/or reproducing information for CD-R (hereinafter referred to also as CD), the numerical aperture of the objective lens is required to have an approximate value of 0.45.
Incidentally, the required numerical aperture mentioned in the present specification is one that is calculated by a spot size d required on the optical information recording medium and wavelength xcex to be used and can be generally expressed by a formula NA=0.83xcex/d.
Information recording surfaces of these optical information recording media are protected by the transparent base boards, and the thicknesses of the transparent base boards are determined by the specifications of each optical information recording medium, to be 0.6 mm for DVD and 1.2 mm for CD system respectively.
If there is no need to consider the interchangeability, the spherical aberration of the pick-up optical system may be designed to be optimum to the thickness of each transparent base board.
However, in order to realize the interchangeability between DVD and CD each having a different thickness of the transparent base board, some means are needed for the above-mentioned correction of the spherical aberration.
For example, in the official gazette of TOKUKAI No. 2000-81566, there is described the objective lens using the diffracted light in the optical pickup apparatus in which DVD and CD are different each other in terms of a wavelength to be used, as an example of means for correction of the spherical aberration.
A power of the diffractive lens can be divided into a power of a refractive section representing a basic aspheric surface and a power of a diffractive section. That is, a degree of freedom in designing can be increased even for a single lens, by introducing the diffractive action. Further, in the diffractive section, there is an effect that the power is proportional to the wavelength which is different from that in the refractive optical system.
By properly allocating the power of the diffractive section and the refractive section, the spherical aberration correction can be performed.
However, in case of recording and/or reproducing information for CD, if the spherical aberration correction is conducted on the outer side of the area covered by the necessary numerical number NA2=0.45, a spot light is stopped down to be too small. Accordingly, in order to use a diaphragm having the common aperture size for DVD and CD, the spherical aberration must be designed so as to let the area outside NA2=0.45 be a flare, when CD is used. Thus, in the so-called diffraction interchangeable lens that can converge the light flux properly for both of DVD and CD, the optical pickup apparatus can be constructed with a single objective lens and a single simple aperture diaphragm, by jointing the diffractive surfaces each being designed optically based on a different concept.
As mentioned above, the diffractive lens power is generally expressed by the sum of the diffractive section power and the refractive section power of the diffractive base surface. However, when a diffractive lens is made of a plastic material, there is observed a distinctive mark that the temperature characteristic and the wavelength characteristic are contrary each other. The reason will be explained as follows.
Let it be considered that the temperature of the diffractive lens has risen. By the increment of temperature, the laser wavelength becomes longer and the over spherical aberration is generated on the diffractive section, while the under spherical aberration is generated on the base refractive section due to the reduction of the refractive index caused by the temperature rise, therefore, the spherical aberrations are produced on the refractive section and the diffractive section wherein both of the spherical aberrations are canceling each other.
Accordingly, to perform the temperature compensation, the absolute values of the spherical aberrations generated on the refractive section and the diffractive section should be close to each other.
On the other hand, when the wavelength is shifted toward the longer wavelength side simply by an unevenness of the laser oscillating wavelength, the over spherical aberration is generated in the diffractive section, but a chromatic aberration in the diffractive section is smaller than the over spherical aberration, resulting in the over spherical aberration in the diffractive section which remains without being cancelled.
Consequently, there is a tendency that, when improving the wavelength characteristics to exceed the certain level, the temperature characteristic begins to go down, while, when improving the temperature characteristics to exceed the certain level, the wavelength characteristic begins to go down, which requires that both of them are balanced.
The invention has been achieved in view of the above-mentioned problems, and an object of the invention is, in the interchangeable objective lens which is using diffraction, to provide with an objective lens, wherein, in addition to the interchangeability between DVD and CD, temperature characteristics is considered, and an objective lens wherein wavelength characteristic is considered and an objective lens wherein these characteristics are well-balanced, and further, to provide with an optical pickup apparatus using these objective lenses.
(1) The optical pickup apparatus described in (1) is represented by an optical pickup apparatus that is provided with a first light source with wavelength xcex1 that records and/or reproduces information by radiating the light flux on the first optical information recording medium with a transparent base board having thickness t1, a second light source with wavelength xcex2 (xcex1 less than xcex2) that records and/or reproduces information by radiating the light flux on the second optical information recording medium with a transparent base board having thickness t2 (t1 less than t2), a light-converging optical system including an objective lens made of a single plastic material that converges the light fluxes radiated from the above-mentioned first and second light sources on the information recording surfaces via the transparent base boards of the above-mentioned first and second optical information recording media, and a diaphragm having the aperture size that is common to both the first and the second optical information recording media, and conducts recording and/or reproducing of information for each optical information recording medium, wherein on at least one side of the objective lens, there exists a diffractive section in which a concentric circular diffractive surface is provided in an effective diameter, and diffraction pitches become smaller gradually in the direction toward the periphery from an optical axis, but the basic aspheric surface of the diffraction is continuous at point h and diffraction pitch increases, when information is recorded on the first optical information recording medium or reproduced from it, the light flux passing through the above-mentioned diffractive section is corrected in terms of aberration to be not more than the diffraction limit in the effective aperture, when information is recorded on or reproduced from the second optical information recording medium, the light flux, passing through the area within a discontinuous section of the spherical aberration in which the step amount is in a range from 7 xcexcm to 40 xcexcm at the point h in the effective aperture in the above-mentioned diffractive section, in a forward optical system, is corrected in terms of the aberration to be the level of the diffraction limit or less, and further, when recording or reproducing of information is performed for the second optical information recording medium, the spherical aberration amount of the ray of light passing through the outermost section of the diaphragm is in a range from 7 xcexcm to 40 xcexcm, therefore, it is possible to adjust the flare amount properly by the discontinuous construction of the above-mentioned diffractive section, and in particular, to secure satisfactorily the temperature characteristics, when recording and/or reproducing of information is performed on the different optical recording media by the use of the same objective lens.
(2) The optical pickup apparatus described in (2) can secure the spot size of the light flux, because the above-mentioned point h satisfies f2(NA2xe2x88x920.03) mmxe2x89xa6hxe2x89xa6f2(NA2+0.03) mm, when the focal length of the objective lens is f2 for recording/reproducing of information on the second optical information recording medium, and the necessary numerical aperture on the second optical information recording medium is NA2.
That is, if the point h is equal to or greater than f2(NA2xe2x88x920.03) mm, a resolving power can be secured for the second optical information recording medium, while if the point h is less than or equal to f2(NA2+0.03) mm, a margin for an inclination error of the second optical information recording medium can be secured.
(3) The optical pickup apparatus described in (3) can secure more satisfactorily the temperature characteristics, because quantity of the spherical aberration of the ray of light passing through the outermost section of the diaphragm is equal to or less than 34 xcexcm, when information is recorded on the second optical information recording medium or reproduced from it.
(4) The optical pickup apparatus described in (4) can secure more satisfactorily the temperature characteristics, because the spherical aberration amount of the ray of light passing through the outermost section of the diaphragm is equal to or greater than 30 xcexcm, when information is recorded on the second optical information recording medium or reproduced from it.
(5) The optical pickup apparatus described in (5) can secure more satisfactorily the wavelength characteristics in addition to the temperature characteristics, because in the forward optical system, there exists the discontinuous section with the step not greater than 9 xcexcm at the point h in the effective aperture of the diffractive section, when information is recorded on the second optical information recording medium or reproduced from it.
(6) The optical pickup apparatus described in (6) is characterized in that, when information is recorded on the second optical information recording medium or reproduced from it, the spherical aberration changes monotonously from the discontinuous section of the spherical aberration to the outermost section of the diaphragm.
(7) The optical pickup apparatus described in (7) is characterized in that, when information is recorded on the first optical information recording medium or reproduced from it, necessary numerical aperture NA1 of the objective lens, the thickness t1 and light source wavelength xcex1 satisfy 0.57 less than NA1 less than 0.63, 0.55 mm less than t1 less than 0.65 mm and 640 nm less than xcex1 less than 670 nm, respectively, and when information is recorded on the second optical information recording medium or reproduced from it, necessary numerical aperture NA2 of the objective lens, the base board thickness t2 and light source wavelength xcex2 satisfy 0.44 less than NA2 less than 0.48, 1.15 mm less than t2 less than 1.25 mm and 770 nm less than xcex2 less than 795 nm, respectively.
(8) The optical pickup apparatus described in (8) is represented by an optical pickup apparatus that is provided with a first light source with wavelength xcex1 that records and/or reproduces information by radiating the light flux on the first optical information recording medium with a transparent base board having thickness t1, a second light source with wavelength xcex2 (xcex1 less than xcex2) that records and/or reproduces information by radiating the light flux on the second optical information recording medium with a transparent base board having thickness t2 (t1 less than t2), a light-converging optical system including an objective lens made of a single plastic material that converges the light fluxes radiated from the above-mentioned first and second light sources on the information recording surfaces via the transparent base boards of the above-mentioned first and second optical information recording media, and a diaphragm having the aperture size that is common to both the first and the second optical information recording media, and conducts recording and/or reproducing of information for each optical information recording medium, wherein on at least one side of the objective lens, there exists a diffractive section in which a concentric circular diffractive surface is provided in an effective diameter, and diffraction pitches become smaller gradually in the direction toward the periphery from an optical axis, but it increases at point h, when information is recorded on or reproduced from the first optical information recording medium, the light flux passing through the above-mentioned diffractive section is corrected in terms of aberration to be not more than the diffraction limit in the effective aperture, in a forward optical system, when information is recorded on the second optical information recording medium or reproducing from it, the light flux passing through the point h in the effective aperture in the above-mentioned diffractive section has the discontinuous section of the spherical aberration, and further the basic aspheric surface of the diffraction at the discontinuous section is discontinuous, therefore, it is possible to adjust the flare amount properly by the discontinuous construction of the above-mentioned diffractive section and the basic aspherical surface, and in particular, to secure satisfactorily the wavelength characteristics, when recording and/or reproducing of information is performed on the different optical recording media by the use of the same objective lens.
(9) The optical pickup apparatus described in (9) is characterized in that the above-mentioned point h satisfies f2(NA2xe2x88x920.03) mmxe2x89xa6h xe2x89xa6f2(NA2+0.03) mm, when f2 represents the focal length of the objective lens for recording or reproducing information for the second optical information recording medium, and NA2 represents the necessary numerical aperture on the second optical information recording medium.
(10) The optical pickup apparatus described in (10) can secure more satisfactorily the wavelength characteristics, because the discontinuous amount of the basic aspheric surface of diffraction is not less than 1 xcexcm and not greater than 10 xcexcm.
(11) The optical pickup apparatus described in (11) can secure more satisfactorily the wavelength characteristics, because the step amount of the discontinuous section of the spherical aberration is not less than 8 xcexcm and not greater than 16 xcexcm.
(12) The optical pickup apparatus described in (12) is characterized in that, when information is recorded on the first optical information recording medium or reproduced from it, necessary numerical aperture NA1 of the objective lens, the thickness t and light source wavelength xcex1 satisfy 0.57 less than NA1 less than 0.63, 0.55 mm less than t1 less than 0.65 mm and 640 nm less than xcex1 less than 670 nm, respectively, and when information is recorded on the second optical information recording medium or reproduced from it, necessary numerical aperture NA2 of the objective lens, the base board thickness t2 and light source wavelength xcex2 satisfy 0.44 less than NA2 less than 0.48, 1.15 mm less than t2 less than 1.25 mm and 770 nm less than xcex2 less than 795 nm, respectively.
(13) The optical pickup apparatus described in (13) is represented by an optical pickup apparatus having therein a light source and a light-converging optical system including an objective lens used for information recording/reproducing for the optical information recording medium, wherein a concentric circular diffractive section is formed on at least one side of the objective lens, and the pitch of the diffractive section becomes gradually narrower monotonously from an optical axis to certain point h, then the pitch of the diffraction increases at both sides of the certain point h, still further, the pitch of the diffraction monotonously decreases in the direction from the point h to the periphery, and the amount of the step in the direction of the depth of the diffractive section at the certain point h is not less than 1 xcexcm and not greater than 10 xcexcm in the direction where the thickness of the outer side of the lens becomes thinner than that of the inner side, and therefore, when information of recording and/or reproducing is performed for the optical information recording medium by the use of the same objective lens, the flare amount can be properly adjusted by the discontinuous construction of the diffractive section, resulting in the compatibility for the wavelength characteristics and the temperature characteristics.
(14) The objective lens described in (14) is represented by an objective lens that is used for an optical pickup apparatus that is provided with a first light source with wavelength xcex1 that records and/or reproduces information by radiating the light flux on the first optical information recording medium with a transparent base board having thickness t1, a second light source with wavelength xcex2 (xcex1 less than xcex2) that records and/or reproduces information by radiating the light flux on the second optical information recording medium with a transparent base board having thickness t2 (t1 less than t2), a light-converging optical system including an objective lens made of a single plastic material that converges the light fluxes radiated from the above-mentioned first and second light sources on the information recording surfaces via the transparent base boards of the above-mentioned first and second optical information recording media, and a diaphragm having the aperture size that is common to both the first and the second optical information recording media, and conducts recording and/or reproducing of information for each optical information recording medium, wherein on at least one side of the objective lens, there exists a diffractive section in which a concentric circular diffractive surface is provided in an effective diameter, and diffraction pitches become smaller gradually in the direction toward the periphery from an optical axis, but the basic aspheric surface of the diffraction is continuous at point h and diffraction pitch increases, when information is recorded on the first optical information recording medium or reproduced from it, the light flux passing through the above-mentioned diffractive section is corrected in terms of aberration to be not more than the diffraction limit in the effective aperture, when information is recorded on or reproduced from the second optical information recording medium, the light flux, passing through the area within a discontinuous section of the spherical aberration in which the step amount is in a range from 7 xcexcm to 40 xcexcm at the point h in the effective aperture in the above-mentioned diffractive section, in a forward optical system, is corrected in terms of the aberration to be the level of the diffraction limit or less, and further, when recording or reproducing of information is performed for the second optical information recording medium, the spherical aberration amount of the ray of light passing through the outermost section of the diaphragm is in a range from 7 xcexcm to 40 xcexcm, therefore, it is possible to adjust the flare amount properly by the discontinuous construction of the above-mentioned diffractive section, and in particular, to secure satisfactorily the temperature characteristics, when recording and/or reproducing of information is performed on the different optical recording media by the use of the same objective lens.
(15) The objective lens described in (15) is characterized in that the above-mentioned point h satisfies f2(NA2xe2x88x920.03) mm xe2x89xa6hxe2x89xa6f2(NA2+0.03) mm, when the focal length of the objective lens is f2 for recording/reproducing of information on the second optical information recording medium, and the necessary numerical aperture on the second optical information recording medium is NA2.
(16) The objective lens described in (16) is characterized in that, when recording or reproducing of information is performed on the above-mentioned second optical information recording medium, the spherical aberration amount of the ray of light passing through the outermost section of the above-mentioned diaphragm is not more than 34 xcexcm.
(17) The objective lens described in (17) is characterized in that the spherical aberration amount of the ray of light passing through the outermost section of the diaphragm is equal to or greater than 30 xcexcm, when information is recorded on the second optical information recording medium or reproduced from it.
(18) The objective lens described in (18) is characterized in that, in the forward optical system, there exists the discontinuous section with the step not greater than 9 xcexcm at the point h in the effective aperture of the diffractive section, when information is recorded on the second optical information recording medium or reproduced from it.
(19) The objective lens described in (19) is characterized in that, when information is recorded on the second optical information recording medium or reproduced from it, the spherical aberration changes monotonously from the discontinuous section of the spherical aberration to the outermost section of the diaphragm.
(20) The objective lens described in (20) is characterized in that, when information is recorded on the first optical information recording medium or reproduced from it, necessary numerical aperture NA1 of the objective lens, the thickness t1 and light source wavelength xcex1 satisfy 0.57 less than NA1 less than 0.63, 0.55 mm less than t1 less than 0.65 mm and 640 nm less than xcex1 less than 670 nm, respectively, and when information is recorded on the second optical information recording medium or reproduced from it, necessary numerical aperture NA2 of the objective lens, the base board thickness t2 and light source wavelength xcex2 satisfy 0.44 less than NA2 less than 0.48, 1.15 mm less than t2 less than 1.25 mm and 770 nm less than xcex2 less than 795 nm, respectively.
(21) The objective lens described in (21) is represented by an objective lens that is used for the optical pickup apparatus that is provided with a first light source with wavelength xcex1, that records and/or reproduces information by radiating the light flux on the first optical information recording medium with a transparent base board having thickness t1, a second light source with wavelength xcex2 (xcex1 less than xcex2) that records and/or reproduces information by radiating the light flux on the second optical information recording medium with a transparent base board having thickness t2 (t1 less than t2), a light-converging optical system including an objective lens made of a single plastic material that converges the light fluxes radiated from the above-mentioned first and second light sources on the information recording surfaces via the transparent base boards of the above-mentioned first and second optical information recording media, and a diaphragm having the aperture size that is common to both the first and the second optical information recording media, and conducts recording and/or reproducing of information for each optical information recording medium, wherein on at least one side of the objective lens, there exists a diffractive section in which a concentric circular diffractive surface is provided in an effective diameter, and diffraction pitches become smaller gradually in the direction toward the periphery from an optical axis, but it increases at point h, when information is recorded on or reproduced from the first optical information recording medium, the light flux passing through the above-mentioned diffractive section is corrected in terms of aberration to be not more than the diffraction limit in the effective aperture, in a forward optical system, when information is recorded on the second optical information recording medium or reproducing from it, the light flux passing through the point h in the effective aperture in the above-mentioned diffractive section has the discontinuous section of the spherical aberration, and further the basic aspheric surface of the diffraction at the discontinuous section is discontinuous, therefore, it is possible to adjust the flare amount properly by the discontinuous construction of the above-mentioned diffractive section and the basic aspherical surface, and in particular, to secure satisfactorily the wavelength characteristics, when recording and/or reproducing of information is performed on the different optical recording media by the use of the same objective lens.
(22) The objective lens described in (22) is characterized in that the above-mentioned point h satisfies f2(NA2xe2x88x920.03) mmxe2x89xa6hxe2x89xa6f2(NA2+0.03) mm, when the focal length of the objective lens is f2 for recording/reproducing of information on the second optical information recording medium, and the necessary numerical aperture on the second optical information recording medium is NA2.
(23) The objective lens described in (23) is characterized in that the discontinuous amount of the basic aspheric surface of diffraction is not less than 1 xcexcm and not greater than 10 xcexcm.
(24) The objective lens described in (24) is characterized in that the step amount of the discontinuous section of the spherical aberration is not less than 8 xcexcm and not greater than 16 xcexcm.
(25) The objective lens described in (25) is characterized in that, when information is recorded on the first optical information recording medium or reproduced from it, necessary numerical aperture NA1 of the objective lens, the thickness t1 and light source wavelength xcex1 satisfy 0.57 less than NA1 less than 0.63, 0.55 mm less than t1 less than 0.65 mm and 640 nm less than xcex1 less than 670 nm, respectively, and when information is recorded on the second optical information recording medium or reproduced from it, necessary numerical aperture NA2 of the objective lens, the base board thickness t2 and light source wavelength xcex2 satisfy 0.44 less than NA2 less than 0.48, 1.15 mm less than t2 less than 1.25 mm and 770 nm less than xcex2 less than 795 nm, respectively.
(26) The objective lens described in (26) is represented by an objective lens that is used for recording/reproducing of information for the optical information recording medium, wherein a concentric circular diffractive section is formed on at least one side of the objective lens, and the pitch of the diffractive section becomes gradually narrower monotonously from an optical axis to certain point h, then the pitch of the diffraction increases at both sides of the certain point h, still further, the pitch of the diffraction monotonously decreases in the direction from the point h to the periphery, and the amount of the step in the direction of the depth of the diffractive section at the certain point h is not less than 1 xcexcm and not greater than 10 xcexcm in the direction where the thickness of the outer side of the lens becomes thinner than that of the inner side, and therefore, when information of recording and/or reproducing is performed for the optical information recording medium by the use of the same objective lens, the flare amount can be properly adjusted by the discontinuous construction of the diffractive section, resulting in the consistency between the wavelength characteristics and the temperature characteristics.
(27) The optical pickup apparatus described in (27) is represented by an optical pickup apparatus for conducting recording and or reproducing information of a first optical information recording medium equipped with a first transparent plate having a thickness of t1 or a second optical information recording medium equipped with a second transparent plate having a thickness of t2 (t1 less than t2), comprising:
a first light source to emit a first light flux having a wavelength of xcex1 to the first optical information recording medium so as to conduct recording and/or reproducing information of the first optical information recording medium;
a second light source to emit a second light flux having a wavelength of xcex2 (xcex1 less than xcex2) to the second optical information recording medium so as to conduct recording and/or reproducing information of the second optical information recording medium;
a converging optical system including a single objective lens made of a plastic material and converging the first light flux onto a recording surface of the first optical information recording medium or the second light flux onto a recording surface of the second optical information recording medium through the respective transparent plate; and
a diaphragm having an aperture used for both of the first and second optical information recording medium;
wherein a diffractive section including a plurality of ring-shaped zones is provided on at least one side surface of the objective lens within an effective diameter of the one side surface, the pitch of the plurality of ring-shaped zones becomes gradually small from the optical axis to a point h within the effective diameter and the pitch increases at the point h;
wherein at the time of conducting recording or reproducing information of the first optical information recording medium, an aberration of the first light flux having passed through the diffractive section is corrected lower than a diffraction limit;
wherein in a forward optical system from the second light source to the recording surface of the second information recording medium at the time of conducting recording or reproducing information of the second optical information recording medium, the second light flux has a spherical aberration discontinuous portion at the point h where a spherical aberration-stepped amount is 10 xcexcm to 30 xcexcm, an aberration of a partial light flux of the second light flux having passed through an inner part of the diffractive section within the point h is corrected lower than a diffraction limit, and a spherical aberration of a light ray of the second light flux having passed through the outermost portion of the aperture of the diaphragm is 40 xcexcm to 70 xcexcm, and
wherein the following conditional formula is satisfied:
0.58 less than NA1 less than 0.62 and 0.48 less than NA2 less than 0.56
where NA1 is a necessary numerical aperture for the first optical information recording medium and NA2 is a necessary numerical aperture for the second optical information recording medium.
(28) The optical pickup apparatus described in (28) is characterized in that the diffractive section of the objective lens has a basic aspheric surface which has a discontinuous portion at the point h and a stepped length of the discontinuous portion is 1 xcexcm to 10 xcexcm.
(29) The optical pickup apparatus described in (29) is represented by an optical pickup apparatus for conducting recording and or reproducing information of a first optical information recording medium equipped with a first transparent plate having a thickness of t1 or a second optical information recording medium equipped with a second transparent plate having a thickness of t2 (t1 less than t2) comprising:
a first light source to emit a first light flux having a wavelength of xcex1 to the first optical information recording medium so as to conduct recording and/or reproducing information of the first optical information recording medium;
a second light source to emit a second light flux having a wavelength of xcex2 (xcex1 less than xcex2) to the second optical information recording medium so as to conduct recording and/or reproducing information of the second optical information recording medium;
a converging optical system including a single objective lens made of a plastic material and converging the first light flux onto a recording surface of the first optical information recording medium or the second light flux onto a recording surface of the second optical information recording medium through the respective transparent plate; and
a diaphragm having an aperture used for both of the first and second optical information recording medium;
wherein a diffractive section including a plurality of ring-shaped zones is provided on at least one side surface of the objective lens within an effective diameter of the one side surface, the pitch of the plurality of ring-shaped zones becomes gradually small from the optical axis to a point h within the effective diameter and the pitch increases at the point h;
wherein at the time of conducting recording or reproducing information of the first optical information recording medium, an aberration of the first light flux having passed through the diffractive section is corrected lower than a diffraction limit;
wherein in a forward optical system from the second light source to the recording surface of the second information recording medium at the time of conducting recording or reproducing information of the second optical information recording medium, the second light flux has a spherical aberration discontinuous portion at the point h where a spherical aberration-stepped amount is 10 xcexcm to 30 xcexcm, an aberration of a partial light flux of the second light flux having passed through an inner part of the diffractive section within the point h is corrected lower than a diffraction limit, and a spherical aberration of a light ray of the second light flux having passed through the outermost portion of the aperture of the diaphragm is 50 xcexcm to 100 xcexcm, and
wherein the following conditional formula is satisfied:
0.62 less than NA1 less than 0.68 and 0.48 less than NA2 less than 0.56
where NA1 is a necessary numerical aperture for the first optical information recording medium and NA2 is a necessary numerical aperture for the second optical information recording medium.
(30) The optical pickup apparatus described in (30) is characterized in that the diffractive section of the objective lens has a basic aspheric surface which has a discontinuous portion at the point h and a stepped length of the discontinuous portion is 1 xcexcm to 20 xcexcm.
(31) The objective lens described in (31) is characterized by an objective lens for use in an optical pickup apparatus for conducting recording and or reproducing information of a first optical information recording medium equipped with a first transparent plate having a thickness of t1 or a second optical information recording medium equipped with a second transparent plate having a thickness of t2 (t1 less than t2)
the optical pickup apparatus is provided with
a first light source to emit a first light flux having a wavelength of xcex1 to the first optical information recording medium so as to conduct recording and/or reproducing information of the first optical information recording medium;
a second light source to emit a second light flux having a wavelength of xcex2 (xcex1 less than xcex2) to the second optical information recording medium so as to conduct recording and/or reproducing information of the second optical information recording medium;
a converging optical system including the objective lens being a single objective lens made of a plastic material and converging the first light flux onto a recording surface of the first optical information recording medium or the second light flux onto a recording surface of the second optical information recording medium through the respective transparent plate; and
a diaphragm having an aperture used for both of the first and second optical information recording medium;
the objective lens, comprising:
a diffractive section including a plurality of ring-shaped zones provided on at least one side surface of the objective lens within an effective diameter of the one side surface,
wherein the pitch of the plurality of ring-shaped zones becomes gradually small from the optical axis to a point h within the effective diameter and the pitch increases at the point h;
wherein at the time of conducting recording or reproducing information of the first optical information recording medium, an aberration of the first light flux having passed through the diffractive section is corrected lower than a diffraction limit;
wherein in a forward optical system from the second light source to the recording surface of the second information recording medium at the time of conducting recording or reproducing information of the second optical information recording medium, the second light flux has a spherical aberration discontinuous portion at the point h where a spherical aberration-stepped amount is 10 xcexcm to 30 xcexcm, an aberration of a partial light flux of the second light flux having passed through an inner part of the diffractive section within the point h is corrected lower than a diffraction limit, and a spherical aberration of a light ray of the second light flux having passed through the outermost portion of the aperture of the diaphragm is 40 xcexcm to 70 xcexcm, and
wherein the following conditional formula is satisfied:
0.58 less than NA1 less than 0.62 and 0.48 less than NA2 less than 0.56
where NA1 is a necessary numerical aperture for the first optical information recording medium and NA2 is a necessary numerical aperture for the second optical information recording medium.
(32) The objective lens described in (32) is characterized in that the diffractive section of the objective lens has a basic aspheric surface which has a discontinuous portion at the point h and a stepped length of the discontinuous portion is 1 xcexcm to 10 xcexcm.
(33) The objective lens described in (33) is characterized by an objective lens for use in an optical pickup apparatus for conducting recording and or reproducing information of a first optical information recording medium equipped with a first transparent plate having a thickness of t1 or a second optical information recording medium equipped with a second transparent plate having a thickness of t2 (t1 less than t2),
the optical pickup apparatus is provided with
a first light source to emit a first light flux having a wavelength of xcex1 to the first optical information recording medium so as to conduct recording and/or reproducing information of the first optical information recording medium;
a second light source to emit a second light flux having a wavelength of xcex2 (xcex1 less than xcex2) to the second optical information recording medium so as to conduct recording and/or reproducing information of the second optical information recording medium;
a converging optical system including the objective lens being a single objective lens made of a plastic material and converging the first light flux onto a recording surface of the first optical information recording medium or the second light flux onto a recording surface of the second optical information recording medium through the respective transparent plate; and
a diaphragm having an aperture used for both of the first and second optical information recording medium;
the objective lens, comprising:
a diffractive section including a plurality of ring-shaped zones provided on at least one side surface of the objective lens within an effective diameter of the one side surface,
wherein the pitch of the plurality of ring-shaped zones becomes gradually small from the optical axis to a point h within the effective diameter and the pitch increases at the point h;
wherein at the time of conducting recording or reproducing information of the first optical information recording medium, an aberration of the first light flux having passed through the diffractive section is corrected lower than a diffraction limit;
wherein in a forward optical system from the second light source to the recording surface of the second information recording medium at the time of conducting recording or reproducing information of the second optical information recording medium, the second light flux has a spherical aberration discontinuous portion at the point h where a spherical aberration-stepped amount is 10 xcexcm to 30 xcexcm, an aberration of a partial light flux of the second light flux having passed through an inner part of the diffractive section within the point h is corrected lower than a diffraction limit, and a spherical aberration of a light ray of the second light flux having passed through the outermost portion of the aperture of the diaphragm is 50 xcexcm to 100 xcexcm, and
wherein the following conditional formula is satisfied:
0.62xe2x89xa6NA1 less than 0.68 and 0.48 less than NA2 less than 0.56
where NA1 is a necessary numerical aperture for the first optical information recording medium and NA2 is a necessary numerical aperture for the second optical information recording medium.
The objective lens described in (31) is characterized in that the diffractive section of the objective lens has a basic aspheric surface which has a discontinuous portion at the point h and a stepped length of the discontinuous portion is 1 xcexcm to 20 xcexcm.
The diffractive section described in this specification means a section where, function to converge or to diverge the light flux by diffraction is provided, by making relief on the surface of the lens. As a form of the relief, there is known a form that is formed on the lens surface as a concentric ring-shaped zone whose center is an optical axis, which looks like a saw-tooth when its section is viewed in a plane including the optical axis.
The objective lens described in this specification means, in a narrow sense, a lens which is arranged to be closest to the optical information recording medium side to face it under the condition that an optical information recording medium is loaded in an optical pickup apparatus, and has a light-converging function, and also means, in a broad sense, a group of lenses which can be moved together with the lens by an actuator in at least the optical axis direction. Herein, the group of the lenses means at least one or more lenses (for example, two lenses)
Accordingly, in this specification, numerical aperture NA of the objective lens closer to the optical information recording medium side (image side) means numerical aperture NA of the lens surface of the objective lens closest to the optical information recording medium.
Further, in this specification, the necessary numerical number NA means a numerical aperture that is prescribed by the specification of each optical information recording medium, or means a numerical aperture of the objective lens having a diffraction marginal power that can obtain a necessary spot diameter for recording or reproducing information, in accordance with the wavelength of the light source in use, for each optical information recording medium.
Further, in the present invention, the definition of the single lens includes a jointed lens in which for example, two lenses are jointed in a single body.
In this specification, the second optical information recording medium means, for example, optical discs of various CDs group such as CD-R, CD-RW, CD-Video and CD-ROM, while the first optical information recording medium means, optical discs of various DVDS group such as DVD-ROM, DVD-RAM, IDVD-R, DVD-RW and DVD-Video.
Moreover, in this specification, the thickness t of the transparent base board includes the case of t=0.